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Re: Backfire

> >JimA=> the exhaust is under positive pressure and will almost never 
> >draw  air INTO itself.  
> I disagree.  the exhaust is not always under positive pressure but
> under a series of pressure pulses.  The are some basic air injection
> systems for emissions that are nothing but a simple valve that
> allows fresh air to enter the exhaust during the low pressure period
> between the pulses.

The systems I'm familiar with all have an air pump.  Who gets by 
without one?

> (And to push this thread into  a different
> direction) This is also how scavenging works on headers...  The
> pulse from one cylinder will help draw the exhaust from another
> cylinder at the collector improving the overall flow...

I don't think this is quite correct.  I believe equal length headers 
allow the pulses from the different cylinders to enter the muffler 
with even timing, which allows each cylinder to operate under the 
same conditions, thus giving more even running.  There is a resonance 
effect with a tuned exhaust, but I really don't know if the pressure 
ever goes below atmospheric.  All it has to do to have a beneficial 
effect is to go below average (at the right time, of course.)

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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